FIG. 33 shows an example of a related art bar code reader and the principle by which it operates. The optical scanner 102 of bar code reader 101 consists of laser light source 103, which is a semiconductor laser element, to emit laser beam 104a or the like; refractive condenser 105 to produce constricted laser beam 104b, scanning unit 106 to scan constricted laser beam 104c on bar code label 108.
When aimed at bar code label 108 on goods 107, laser beam 104c is absorbed by the black bars in bar code 109 and reflected by the white spaces. Optical reader unit 117 converts reflected laser beam 104d, which strikes photodetector unit 110, into an analog signal 111 which corresponds to the pattern of bar code 109. This signal is amplified by video amplifier 112. Amplified signal 113 is converted to digital signal 115 by A/D converter 114, decoded by decoder 116, and output.
It is well known that the width of a narrow bar on the bar code label must be matched with the resolution capability of the reader to optimize the rate at which bar codes can be read by a bar code reader. If the resolution capability is lower than the width of a bar, the reading rate will naturally decrease; on the contrary, if the resolution capability is excessive with respect to the width of the bars, the reading rate will likewise decrease. This is because a reader with too high a resolution capability will detect irregularities such as rough edges on the bars, voids (holes within the bars) and spots in the spaces between the bars.
However, the resolution capability of a bar code reader depends on the diameter of the beam, which in turn depends on the distance from the bar code reader 101 to the object which is to be read (the reading distance). Thus, the reading rate for bar codes is limited on its upper end. Let us use the example of the optical device in optical scanner 102, pictured in FIG. 34. In this device, the region which contains the focal point F will have high resolution; the region adjacent to focal point F will have medium resolution; and the region furthest away will have low resolution. If the width of bar code is very thin or if the bar code on goods is positioned at a long reading distance, the range in which the code can be read will be extremely short. This is why existing bar code readers require that the position of the bar code label be moved into the high resolution region, and the operator is required to search for that region by moving the goods around.
Another existing bar code reader applies a pulse drive to a laser light source over a drive time T.sub.1, so as to divide the bar code lines into numerous segments. By accepting reflected signals over a corresponding period of time, a bar code reader can read a code with accuracy. For example, FIG. 35(a) shows an enlarged segment (Segment Y) of bar code label 108 shown in FIG. 36. The segment contains two lines 109a. As shown in FIG. 35(b), the drive circuit for the laser light source outputs pulsed drive current 117 with a period T.sub.1, which corresponds to the narrower width of lines 109a. The pulse drive causes the laser light source to emit light. The processing unit which receives the optical signal accepts signal 118 from a photodetector unit for a period T.sub.1 corresponding to a single pulse of drive current 117. To prevent reading errors, each time it reads a label the reader scans a laser beam a number of times, so the reliability of signal detection is quite high.
When the object to be detected (the bar code label) is in the position where the light from the optical scanner can be focused (high resolution area), the light will be absorbed by lines 119a and reflected by the white spaces. A reflected signal 118 with a proper S/N ratio, as shown in FIG. 35(c), will be obtained. If, on the other hand, the object is far from the proper location or close to it but not in it (e.g., in the low resolution area), the diameter of the light beam will increase, and reflected signal 118 will have a low S/N ratio, as shown in FIG. 35(d). Thus, this type of bar code reader also requires that label 108 be moved to the high resolution area, so the operator must search for that area by moving the goods around.
These same kind of reading problems may occur photoelectric sensors as long as the sensor elements are positioned in a low resolution area.